For typical home use, I'd go with a 3/4hp unit. If you intend to grind lots of tough stuff, eg bones, then I'd get a 1hp. I just installed a new Insinkerator 3/4hp unit. Got it at Lowes. Apparently Lowes has some models made just for them, as I didn't see them anywhere else. One thing I liked about these were they had nice polished stainless flanges that go in the sink drain. By comparison, the Sears units had dull stainless that looks like crap in a nice sink. As far as sound, it's pretty quiet compared to the old one it replaced.
One think I did learn a little late, was the Insinkerator doesn't have a removable rubber baffle in the opening. The opening is also smaller than the old GE unit I replaced. Those two things make it harder to get your hand inside to retrieve anything.